The present invention relates to a thermostatically controlled solid fuel burning stove and to a thermostat for such a stove.
Because of the quickly rising prices, or expected shortages, or both, of petroleum and natural gas, people are seeking alternative energy sources, especially for the purpose of heating homes. Fuels such as wood and coal are regaining a measure of their former popularity. As a result, there is a great demand for stoves that will produce heat from wood, coal, etc. as efficiently and safely as possible.
It is highly desirable to control the temperature at which combustion occurs in the stove. When the combustion occurs within the optimum range for heating a room to a desired temperature, the inhabitants of the room will be as comfortable as possible; clearly this will not be the case if the combustion occurs at a higher or a lower temperature or if its temperature oscillates widely. If the combustion temperature is too high, in addition to causing discomfort, the stove will consume more fuel than is necessary. In addition, with the stove at a high temperature, there is a danger of chimney fire due to high stack temperatures which are produced. Finally, an unnecessarily hot stove increases the danger of a person in the room suffering a severe burn from accidentally touching it or the stove igniting something flammable which is near the stove.
If the stove combustion temperature is too low, on the other hand, efficiency is again reduced, excessive CO is formed and there is a danger that the fire will go out, presenting the inconvenience of having to restart it.
Wood burning stoves equipped with thermostats which attempt to maintain the combustion temperature at the desired level are known. Such a stove is disclosed in U.S. Pat. No. 4,117,824, issued Oct. 3, 1978, to McIntire et al. In the McIntire et al. patent, a damper is mounted in an air intake and can be rotated about its edge to control the quantity of air supplied to the stove combustion chamber. This controls the combustion temperature. A bimetallic coil is provided outside the air intake duct. One end of the coil is connected to the damper, and the other end of the coil is connected to a control knob. When the coil heats as a result of a rise in the stove temperature, it rotates and moves the damper in the closed direction. Conversely, a drop in the stove temperature as measured by the bimetallic coil results in the damper being opened further. The position of the damper can also be controlled manually by turning the control knob.
The combustion temperature in wood burning stoves with known thermostats typically exhibits oscillations of a very large amplitude, often reaching 70-300.degree. F. This is because known thermostats typically are not sufficiently sensitive to changes in the stove temperature. Such thermostats generally respond to temperature changes only very slowly and display a response that is very small in magnitude. As a result, by the time the thermostat reduces the air supply responsive to a temperature rise, the temperature has risen very high, and the amount of the reduction in the air supply is correspondingly large. Because of this, the combustion temperature falls dramatically. Due to its relative insensitivity, however, the thermostat detects the temperature drop only after the temperature has fallen as much as 70.degree.-300.degree. F. below its peak. The thermostat responds by drastically increasing the air supply, allowing the combustion temperature to rise very quickly to a new peak. As a result, the stove temperature oscillates wildly, instead of remaining at a stable level.
A stove controlled by such a thermostat thus reaches temperatures both substantially higher and substantially lower than the optimum combustion temperature and accordingly presents many of the risks and suffers from many of the drawbacks of manually controlled stoves.